1. Field of the Invention
The present invention relates generally to local area networks, and more specifically to a bridge for interconnecting local area networks at OSI (Open System Interconnection Reference Model) physical and data link layers.
2. Description of the Related Art
For interconnecting local area networks at the OSI lower two layers, a device called "layer-2 relay", or "bridge" is used. The conventional LAN bridge includes an address filter associated with one of the local area networks for allowing only those packets destined to local area networks other than the associated local area network to be forwarded from the bridge through an output port. Thus, packets destined to the home local area network are rejected by the bridge. Specifically, the address filter has a source address detector, a destination address detector and a registration table. The source address detector extracts a source address from a packet arriving from the associated local area network, the extracted source address being stored into the registration table. The destination address detector extracts a destination address from the packet. The registration table is then searched for an address identical to the extracted destination address. If the same destination address is stored in the registration table, the transmission of the packet from the bridge is prevented. If the same destination address is not detected in the registration table, it is recognized that the packet is destined to a local area network other than the associated network, and the packet is forwarded from the bridge. Such LAN bridges are usually provided one for each direction of transmission between two LANs so that all the LANs are interconnected to allow the terminals of each LAN to access the terminals of other networks. Therefore, the packets directed to a particular LAN are also transported to other LANs as well. For purposes of network management and maintenance, the bridge is also designed to relay broadcast packets specified by the protocol of the OSI network layer. Since the OSI network layer recognizes all the interconnected networks as a single entity, the broadcast packet from each LAN is transported to all the other local area networks.
Recent advances in technologies permit transmission of high-priority packets such as those transmitted using multimedia application software. However, such high priority packets are vulnerable to delays which would be caused by the transmission of low-priority packets destined to other local area networks over the common network facility as well as by broadcast packets.
Japanese Provisional Patent Publication (Tokkai-Hei) 1-189252 discloses a LAN bridge that prevents packets from being forwarded onto the common facility of an interconnected LAN network by appending a special code to each packet. However, protocols must be altered to prevent transfer of such packets.